I used to have this problem.I have found 2 solutions one straight forward and easy, the other needing more skill! First the easy solution. As you're probably aware, the strong sun is making the door warm so its expanding a bit, and therefore its a tighter fit in the frame. The first thing I would do is check for dirt deposits all the way round the frame, especially in the sill at the bottom where dirt, soil etc accumulates and over time can build up quite a lot. This can be enough to prevent the door closing when its hot. Cleaning this out thoroughly and keeping it clean, may be all that is needed, so I would try this first! It may also be a good idea to oil or grease any hinges and catches and contact points. If this doesn't work then its possible the door has dropped a bit and needs adjusting. I'm sorry I can't help with this as this is where I call someone in to help, but I'm sure some one will be along soon with more expertise! Hope this helps.
Jenny, July 2009